The present invention relates to the correction of abrasion in the case where a wearing tool such as a grind stone, a buff, a wire brush and the like is used for a robot.
In the case where a robot is used to carry out a burring work, when a grind stone is used as a tool, the grind stone gradually becomes worn out to reduce its diameter. It is therefore necessary to correct the reduced portion of the diameter by some kind of methods.
In an automatic machine which merely moves in a linear fashion, a reduced portion of the diameter is measured by a sensor, and the grind stone is parallel-moved in one direction by the amount measured whereby the correction of the worn amount may be corrected.
However, in the apparatus which involves complicated movements in a three-dimensional fashion such as a robot, the correction of the diameter cannot be carried out merely by parallel-moving the grind stone in one direction on an absolute coordinate on a normal robot control point.
Therefore, methods described below have been hitherto used to correct the abrasion.
(1) Method utilizing a force sensor
When a grind stone becomes worn out, a pressing pressure to work varies. a position of a robot is adjusted while detecting a pressing pressure by use of a force sensor so that the pressing pressure to the work is constant, whereby correction of abrasion of the grind stone is effected.
(2) Method for attaching a tool holder capable of sliding a wearing tool to a wrist of a robot
A tool holder capable of sliding a grind stone is attached to a robot wrist, and the worn amount is detected by a sensor, after which the amount is slidably moved.
Then, a robot is operated so that the slid direction is always at a given angle with respect to a work surface, whereby correction of abrasion of the grind stone is effected.
Alternatively, there is a method, to which the aforementioned method is applied, for mounting a device disclosed in Japanese Patent Application Laid-Open No. 9862/1987 to a robot wrist.
(3) Method in which the worn amount of a tool is calculated by position detection and storage function of a robot, and the worn portion is shifted on rectangular coordinates of the robot (the absolute coordinates set to the robot body).
In this method, as disclosed in Japanese Utility Model Application Laid-Open No. 187905/1985 and Japanese Patent Application Laid-Open No. 188095/1986, a wearing work tool is provided on a wrist portion of a robot and an on-off type sensor is provided externally to detect the worn amount of the tool. The robot causes the wearing work tool in a predetermined direction from a reference position toward the sensor, the present position of the robot when the sensor is turned on is stored, the worn amount of the wearing work tool is calculated from a difference between the reference position and the present position when the sensor is turned on, and the worn amount of the work tool is parallel-shited in one direction on the rectangular coordinates of the robot (the absolute coordinates set to the robot body) whereby correction of abrasion of the tool is effected.
While the above-described method disclosed in Utility Model Application Laid-Open No. 187905/1985 and Japanese Patent Application Laid-Open No. 188095/1986 is effective in the method for decting the worn amount of the work tool, there is a problem in the abrasion correction method itself.
More specifically, let a be the work tool prior to abrasion and let b be the work tool after abrasion, if work is a flat work W (a material to be cut) on a plane of rectangular coordinates H of a robot (the absolute coordinates set to the robot body), the method is recognized to be effective. However, for those in which the work surface varies in a three-dimensional fashion or with respect to the operation on the circumference, the robot empolying the above-described method of correction cannot be applied since the correction method is one direction of the rectangular coordinates H of the robot. That is, in the case where a polygonal Work W' is cut by a circular work tool (for example, such as a grind stone) as shown in FIG. 2, a robot locus after abrasion has been corrected (after shifting) becomes deviated, posing a problem in that the work tool does not contact with the work W', rendering the operation itself impossible.
In FIG. 2, S is the worn amount (which is equal to the shifted amount), and arrow f is the shifting direction.
Furthermore, the aforementioned method using a force sensor involves the problem in that the sensor itself is expensive, the problem of delay in response of a sensor, and the problem in reliability in adverse environment in burring. Moreover, in the aforementioned method in which a slide device is mounted, the wrist portion becomes large, the apparatus which can withstand high loads and shocks and which accurately performs locating is expensive and there involves a problem in reliability in the adverse environment.